My invention relates to an apparatus for separating and feeding stacked sheets for a pneumatic sheet separating and conveying mechanism in a sheet feeder of a sheet processing machine.
A suction head in a sheet feeder for a sheet processing machine is described in German Patent 22 20 353, which is provided with separating suction for lifting a sheet from a stack and with conveying suction, which is needed to feed the sheets in the direction of other sheet conveying means. These devices, and other devices having the same basic principle, separate and feed the sheets according to a rear edge principle in such a way that the upper sheet of the stack is preloosened at its rear edge by one or more blower nozzles and the uppermost sheet of the stack is gripped at its rear edge by a mechanical spring-operated separating suction device and/or by a separating suction device operating according to the pressure difference principle. The sheets are thereby moved forward with their rear edges for the purpose of a reliable separation by a skimmer element. After that, a separating member swings into the gap formed between the lifted sheet and the remaining stack and presses on the sheet lying under it, so that, as a result of the applied force, the sheets remaining in the stack are fixed in their position. Simultaneously compressed air is blown under one end of the lifted sheet, since as large as possible a loosening of the sheet should be attained. During the blowing-under stage, the sheet is gripped by conveying suction located downstream of the separating suction and simultaneously released by blowing air from the separating suction devices and is conveyed by the suction conveyer system with the conveying suction to the belt transport system and transferred to it.
In this process there is a series of connections which determine the individual process stages of the described mechanism and the time interval and arrangement for return to their starting configuration. Thus, the separating suction and also the conveying suction only grasp the new following sheet, when the preceding sheet has passed into the working range of some suction device. The under blowing of the sheet can occur only in the time interval between activation of the separating member and the time at which air is blown from the separating suction devices. No reliable separation of large sheets of the stack can be attained in the processing of large sheets and/or of sheets with a length which is large in the feed direction in this short time interval, so that the output of the known apparatus is limited and the structure of the sheet feeder machines no longer attains the high output parameters.
To avoid this difficulty the time of blowing under the sheets was indeed extended by the additional arrangement of suction units connected operationally with the conveying suction mechanisms at the level of the sheet rear edges, so that the sheet separation is somewhat improved and the sheet feed is supported by the air stream acting in the feed direction, however the output increase of such an apparatus made possible by increasing the blowing-under time is not sufficient, since, because of the arrangement additional suction devices in the already highly loaded conveying system, its performance limit regarding the allowed following cycles is limited. Moreover the reliability of operation is reduced during processing stacks of sheets with undulating surfaces by suction devices distributed over the larger surface areas.